A strain gage type pressure transducer operates to convert a physical displacement into an electrical signal. This type of pressure transducer is well known in the art and generally comprises a deflectable member having one or more piezoresistive elements fabricated thereon. When a force is applied to the deflecting member of this transducer, a voltage is placed across the piezoresistive elements and as the deflecting member bends in response to applied pressure, a resistance change in the piezoresistive elements results in a change in the current flowing through these elements.
In the case of the typical monolithic pressure transducer, the deflecting member comprises a thin silicon diaphragm or cantilever beam into which piezoresistors are diffused or implanted and then connected either internally or externally to form a Wheatstone bridge circuit. Since the piezoresistors are integrally formed on the surface of the deflecting member this configuration acts to effectively amplify the sensitivity of the piezoresistive elements to the force applied to the deflecting member.
Such an integral configuration has the benefit of temperature operation to almost 350.degree. F. Operation at or above this temperature, however, causes the isolation between the sensors and the silicon substrate to deteriorate. The deterioration is caused by thermally generated carriers which serve to short circuit the sensors to the silicon substrate. This problem was essentially solved in the prior art by dielectrically isolating the sensors from the deflecting member. For example, see U.S. Pat. Nos. 3,800,264 and 3,930,823 both of which are entitled HIGH TEMPERATURE TRANSDUCERS AND HOUSING INCLUDING FABRICATION METHODS and both of which are by A. D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc. the assignee herein.
The devices discussed in the above-named patents, provide dielectric isolation between the sensor network and the deflecting member or force collector. These devices are capable of operating at temperatures in excess of 500.degree. C. Above 600.degree. C., however, the silicon sensing network as well as the silicon deflecting member or force collector, undergo significant plastic deformation rendering the device useless as a pressure transducer. This problem was addressed and solved in the prior art by employing silicon carbide (SiC) both as a sensor and as the force collector. For example, see U.S. Pat. No. 5,165,283 entitled HIGH TEMPERATURE TRANSDUCERS AND METHOD OF FABRICATING THE SAME EMPLOYING SILICON CARBIDE by A. D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc. the assignee herein. This patent discloses a pressure transducer fabricated from SiC wherein the sensors are dielectrically isolated from the force collector by means of a p-n junction. The pressure transducer described therein is capable of operating at extremely high temperatures in excess of 600.degree. C.
Even though the pressure transducers described in U.S. Pat. No. 5,165,283 are capable of operating at high temperatures, it is difficult to obtain very accurate pressure measurements because of the large variation of sensor output with temperature.
It is well known that most semi-conductor materials exhibit a gage factor which decreases as a function of temperature. The gage factor of a material is the measure of the change of resistance of that material with applied strain. It is desirable for a piezoresistive transducer employing a Wheatstone bridge to have an output which is independent of temperature. This requires a bridge voltage which will increase at a rate that equals the rate at which the gage factor decreases.
If for instance, the TCR (Temperature Coefficient of Resistance) of the Wheatstone bridge is positive and greater than the negative TC of gage factor then, temperature compensation can be obtained by placing a resistor having a zero TCR in series with the Wheatstone bridge and placing a constant voltage across the series combination of the resistor and the bridge.
In silicon carbide, however, not only is the TC of the gage factor negative, but for most resistivities, the TCR is also negative; or has a low positive value.
It is, therefore, an object of the present invention to provide a pressure transducer which employs a plurality of piezoresistive sensors coupled in a Wheatstone bridge configuration and a network of resistors coupled in series with the Wheatstone bridge wherein the resistor has a negative TCR, the value of which is greater than the positive TCR value of the Wheatstone bridge.